Nearly 16 months after its debut, a pricey ploy to run the L train on autopilot is still hamstrung by glitches that cause aggravating delays for riders and multimillion-dollar repairs for the MTA, according to train operators and union leaders.

Yet despite the lousy track record, the cash-strapped agency is gearing up to spend $453 million to create another “robotrain,” on the No. 7 line in 2015.

“It’s trying to fit new technology into an old system,” said William Henderson, executive director of the Permanent Citizens Advisory Committee to the MTA. “There is some skepticism . . . about why we are doing this.”

The newfangled system, which cost more than $340 million to rig on the L line, has computerized signals that automatically detect and pilot the train. But NYC Transit’s old infrastructure sometimes causes the new signals to short out and make the brakes seize up, resulting in delays from 15 minutes to nearly an hour, according to those who work the line.

“There’s a fluke in the system. It’s constantly happening,” said one train operator.

In the last three weeks, the L has suffered from “mechanical problems” at least 10 times, two of them on Friday alone, according to NYC Transit information. Those incidents also include a hot, sweaty wait during rush hour on July 8, where the crowding nearly caused a riot.

“It was a nightmare from hell,” said Brooklynite James Cox, 26, who was caught on the platform for a half-hour.

Last week, the MTA board agreed to shell out $2 million to keep contractors on the dime to deal with the roughly 100 glitches the line averages a year, according to documents. MTA mechanics lack “the specialized training” to conduct the work. This after the autopilot trains were supposed to cut down on staffing costs.

Transit spokesman Charles Seaton said a component in the system has experienced a “higher than expected failure rate,” and that the No. 7 line will use a different contractor and train cars. Still, Seaton said the L line’s system “is working,” and the autopilot runs fine on about two dozen trains a day.